Joining together consecutive date validity intervals

Question

I have a series of records containing some information (product type) with temporal validity.

I would like to meld together adjacent validity intervals, provided tha

Answer 1

This is a pretty complicated set of steps, but it's the way that I solved a similar problem:

-- Sample Data
CREATE TABLE AdjacentValidity 
  (
RowID INT IDENTITY(1,1) NOT NULL,
Product VARCHAR(1) NOT NULL,
Start_Date DATETIME NOT NULL,
End_Date DATETIME NOT NULL
  )

INSERT INTO AdjacentValidity (Product, Start_Date, End_Date)

SELECT 'A', '7/1/2013', '7/31/2013' UNION
SELECT 'A', '8/1/2013', '8/31/2013' UNION
SELECT 'A', '9/1/2013', '9/30/2013' UNION
SELECT 'B', '10/1/2013', '10/31/2013' UNION
SELECT 'B', '11/1/2013', '11/30/2013' UNION
SELECT 'A', '12/1/2013', '12/31/2013' UNION
SELECT 'A', '1/1/2014', '1/31/2014' UNION
SELECT 'A', '2/1/2014', '2/28/2014' UNION
SELECT 'A', '3/1/2014', '3/31/2014'


-- Modify the sample data to include necessary tags
CREATE TABLE #RawData
  (
    RawData_ID INT IDENTITY(1,1) NOT NULL,
    Product VARCHAR(1) NOT NULL,
    Start_Date DATETIME NOT NULL,
    End_Date DATETIME NOT NULL,
    isFirstOccurrence BIT NULL,
    isLastOccurrence BIT NULL,
    isFirstInstance BIT NULL,
    isLastInstance BIT NULL
  )

-- Load and flag first occurrences of a natural key
INSERT INTO #RawData
  (
    Product,
    Start_Date,
    End_Date,
    isFirstInstance
  )
SELECT 
    Product,
    Start_Date,
    End_Date,
    CASE WHEN ROW_NUMBER() OVER
      (
        --PARTITION BY 
        ORDER BY Start_date
      ) = 1 THEN 1 ELSE 0 END AS isFirstOccurrence
FROM AdjacentValidity

-- update to flag the last sequential instance of a particalar data set, and the last     occurrence of a natural key
UPDATE a
SET 
    a.isLastInstance = 
     CASE 
      WHEN 
        a.Product <> b.Product OR 
        DATEADD(m, 1, a.Start_Date) <> b.Start_Date  OR
        b.RawData_ID IS NULL
      THEN 1 
      ELSE 0 
     END,
    a.isLastOccurrence = 
     CASE
      WHEN 
        b.RawData_ID IS NULL
      THEN 1 
      ELSE 0 
     END 
FROM 
    #RawData a
     LEFT JOIN
    #RawData b ON 
        b.RawData_ID = a.RawData_ID + 1 --AND 
        --b. = a.

--  flag first sequential instance of a particular data set
UPDATE b
SET 
    b.isFirstInstance = 
     CASE 
      WHEN 
        a.isLastInstance = 1
      THEN 1 
      ELSE 0 
     END 
FROM 
    #RawData a
     LEFT JOIN
    #RawData b ON 
        b.RawData_ID = a.RawData_ID + 1 --AND 
        --b. = a.


-- reduce the records to only those that are the first or last occurrence of a     particular data set
CREATE TABLE #UniqueData 
  (
    [UniqueData_ID] [int] IDENTITY(1,1) NOT NULL,
    Start_Date DATETIME NOT NULL,
    End_Date DATETIME NOT NULL,
    Product VARCHAR(1) NULL,
    isFirstOccurrence BIT NULL,
    isLastOccurrence BIT NULL,
    isFirstInstance BIT NULL,
    isLastInstance BIT NULL
  ) 

INSERT INTO #UniqueData
  (
    Start_Date,
    End_Date,
    Product,
    isFirstOccurrence,
    isLastOccurrence,
    isFirstInstance,
    isLastInstance
  )

SELECT 
    Start_Date,
    End_Date,
    Product,
    isFirstOccurrence,
    isLastOccurrence,
    isFirstInstance,
    isLastInstance
FROM 
    #RawData 
WHERE 
    isFirstOccurrence = 1 OR
    isFirstInstance = 1 OR
    isLastInstance = 1
ORDER BY RawData_ID, Start_Date




-- combine the first and last occurrences in any given sequence into a single row
SELECT 
    a.Start_Date,
    ISNULL(b.Start_Date, a.End_Date) End_Date,
    a.Product
FROM 
    #UniqueData a 
     LEFT JOIN
    #UniqueData b ON 
        b.UniqueData_ID = a.UniqueData_ID + 1 AND
        --b. = a. AND
        a.isLastInstance <> 1
WHERE a.isFirstInstance = 1 or a.isFirstOccurrence = 1
ORDER BY a.UniqueData_ID



-- clean up
/*
DROP TABLE AdjacentValidity
DROP TABLE #RawData
DROP TABLE #UniqueData
*/